CN110662600A

CN110662600A - Microwave system and method for graphene production

Info

Publication number: CN110662600A
Application number: CN201780091060.0A
Authority: CN
Inventors: 林怡君; 阿茹娜·扎姆; 张博增
Original assignee: Nanotek Instruments Inc
Current assignee: Nanotek Instruments Inc
Priority date: 2017-04-19
Filing date: 2017-06-09
Publication date: 2020-01-07
Anticipated expiration: 2037-06-09
Also published as: KR20190137152A; JP2020517561A; WO2018194696A1; US20180305213A1; JP7086099B2; KR102506112B1; CN110662600B; US10822239B2

Abstract

There is provided a process for producing graphene from microwaveable unexpanded graphite or graphitic carbon, the process comprising: (a) feeding a powder of the microwaveable material onto a non-metallic solid substrate, wherein the powder is in the shape of a ribbon having a first ribbon width and a first ribbon thickness; (b) moving the ribbon-shaped powder into a microwave applicator chamber, the microwave applicator chamber comprising a microwave power zone having a microwave application width (not less than the first ribbon width) and a microwave penetration depth (not less than the first ribbon thickness) such that the entire ribbon-shaped powder receives and absorbs microwave power at a sufficient power level for a sufficient time to puff and separate the powder to produce graphene sheets; and (c) removing the graphene sheets from the microwave chamber, cooling the graphene sheets, and collecting the graphene sheets in a collection container or for subsequent use.

Description

Microwave system and method for graphene production

Cross Reference to Related Applications

This application claims priority to U.S. patent application No. 15/491,714, filed on 19/4/2017, which is incorporated herein by reference.

Technical Field

The present invention relates to systems and methods for producing graphene materials from intercalated graphite, intercalated graphitic carbon, graphite oxide carbon, and graphite fluoride.

Background

A single layer graphene sheet consists of carbon atoms occupying a two-dimensional hexagonal lattice. Multi-layer graphene is a platelet composed of more than one hexagonal plane of carbon atoms. Few-layer graphene sheets or platelets refer to graphene materials having 2-10 hexagonal carbon atom planes. Individual single-layer graphene sheets and multi-layer graphene platelets are collectively referred to herein as nano-graphene platelets (NGPs) or graphene materials. NGPs include pristine graphene (substantially 99% carbon atoms), micro-oxidized graphene (< 5% oxygen by weight), micro-fluorinated graphene (< 5% fluorine by weight), fluorinated graphene (> 5% fluorine by weight), other halogenated graphene, hydrogenated graphene, and chemically functionalized graphene.

Graphene has been found to have a range of unusual physical, chemical and mechanical properties. For example, graphene was found to exhibit the highest intrinsic strength and highest thermal conductivity of all existing materials. Although practical electronic device applications of graphene (e.g., replacing Si as a backbone in transistors) are not expected to occur within the next 5-10 years, its application as a nanofiller in composite materials and as an electrode material in energy storage devices is imminent. The availability of large numbers of processable graphene sheets is crucial for the successful development of graphene composites, energy and other applications.

Our research group first discovered Graphene worldwide [ b.z.jang and w.c.huang, "Nano-scaled Graphene Plates ]", U.S. patent application No. 10/274,473 filed 10/21 2002; now U.S. Pat. No. 7,071,258(07/04/2006) ]. Recently, we reviewed methods for producing NGP and NGP Nanocomposites [ Bor z. jang and a Zhamu, "Processing of Nano Graphene Plates (NGPs) and NGP Nanocomposites: a Review [ Processing of Nano Graphene Platelets (NGP) and NGP Nanocomposites: review ] ", J.materials Sci. [ journal of materials science ], 43(2008) 5092-5101. Four major prior art methods have been followed to produce NGP. Their advantages and disadvantages are briefly summarized as follows:

the method comprises the following steps: chemical formation and reduction of Graphite Oxide (GO) platelets

The first method (fig. 1) entails treating natural graphite powder with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a Graphite Intercalation Compound (GIC) or indeed Graphite Oxide (GO). The graphite has an interplanar spacing (L) of about 0.335nm of graphene prior to intercalation or oxidation_d＝¹/₂ d₀₀₂0.335 nm). In the case of intercalation and oxidation processes, the inter-graphene spacing increases to values typically greater than 0.6 nm. This is the first expansion stage that the graphite material undergoes during this chemical route. The resulting GIC or GO is then subjected to further expansion (often referred to as swelling) using a thermal shock exposure process or a solution-based sonication assisted graphene layer swelling process.

In the thermal shock exposure process, the GIC or GO is exposed to an elevated temperature (typically 800 ℃ -1,200 ℃) for a short period of time (typically 30 seconds to 3 minutes) to expand the GIC or GO to form expanded graphite (or further expanded graphite), typically in the form of "graphite worms" comprised of graphite flakes that are still interconnected with one another. This thermal shock procedure can produce some separate graphite flakes or graphene sheets, but typically most of the graphite flakes remain interconnected. Typically, the expanded graphite or graphite worms are then subjected to flake separation using air milling, mechanical shearing, or sonication in water. Thus, method 1 basically requires three different procedures: first expansion (oxidation or intercalation), further expansion (or "swelling"), and separation.

In a solution-based separation process, expanded or expanded GO powder is dispersed in water or an aqueous alcohol solution, which is subjected to sonication. It is important to note that in these processes, sonication is used after intercalation and oxidation of the graphite (i.e. after the first expansion) and typically after thermal shock exposure of the resulting GIC or GO (after the second expansion). Alternatively, GO powders dispersed in water are subjected to ion exchange or lengthy purification procedures in such a way that the repulsive forces between ions present in the inter-planar spaces prevail over the van der waals forces between graphene, resulting in graphene layer separation.

Thermal expansion requires the use of a high temperature furnace preset to a temperature typically from 800 ℃ to 1,200 ℃, and is therefore a highly energy-consuming process. Furthermore, thermal expansion based on ovens alone typically results in the formation of primarily graphite worms, which must be subjected to further mechanical shearing to produce isolated graphene sheets. There is a need for a more energy efficient and efficient method of producing ultra-thin graphene sheets (predominantly single or few layers).

The method 2 comprises the following steps: direct formation of pristine nano-graphene platelets

In 2002, our research team successfully isolated single and multilayer graphene sheets from partially carbonized or graphitized polymeric carbons obtained from polymer or pitch precursors [ b.z.jang and w.c. huang, "Nano-scaled graphene plates ]", U.S. patent application No. 10/274,473 filed 10/21/2002; now U.S. Pat. No. 7,071,258(07/04/2006) ].

Mack et al [ "Chemical manufacturing of nanostructured materials ]", U.S. Pat. No. 6,872,330 (3.29.2005) ] developed a process that involved intercalating graphite with a potassium melt and contacting the resulting K-intercalated graphite with alcohol to produce severely expanded graphite containing NGP. The process must be carefully carried out in a vacuum or very dry glove box environment because soda metals such as potassium and sodium are extremely moisture sensitive and present an explosion hazard. This method is not suitable for mass production of NGP.

The method 3 comprises the following steps: epitaxial growth and chemical vapor deposition of nano-graphene sheets on inorganic crystal surfaces

Small-scale production of ultra-thin graphene sheets on a substrate can be achieved by epitaxial growth based on thermal decomposition and laser desorption-ionization techniques. Epitaxial films of graphite having only one or a few atomic layers are of technical and scientific importance due to their characteristic features and great potential as substrates for devices. However, these methods are not suitable for the mass production of isolated graphene sheets for composite and energy storage applications.

The method 4 comprises the following steps: bottom-up method (Synthesis of graphene from Small molecule)

Yang et al [ "Two-dimensional Graphene Nano-ribbons ]", j.am.chem.soc. [ american chemical society ]130(2008)4216-17] synthesized Nano-Graphene sheets up to 12nm in length using the following method, starting with a Suzuki-Miyaura coupling of 1, 4-diiodo-2, 3,5, 6-tetraphenyl-benzene with 4-bromobenzoic acid. The resulting hexaphenylbenzene derivatives were further derivatized and ring-fused into small graphene sheets. This is a slow method to date to produce very small graphene sheets.

Microwaves have been used to assist in the production of expanded graphite (graphite worms), graphite nanoplatelets (or expanded graphite), and graphene. For example, drzal et al use microwaves as a heating means to produce exfoliated graphite nanoplatelets [ "expanded graphite and Products Produced from expanded graphite," U.S. publication nos. 20040127621 (7/1/2004) and US 20060148965 (7/6/2006) ]. In this prior art process, natural graphite is intercalated and oxidized by using a strong acid and an oxidizing agent. After the intercalation/oxidation process, the natural graphite is rinsed, dried and recovered from the liquor. The dry powder is GIC or GO, which is then subjected to microwave heating to obtain expanded graphite nanoplatelets, rather than thin graphene sheets (i.e., single or few layer graphene). Furthermore, typically some GICs are not expanded or puffed and, therefore, the process requires an additional step to separate (isolate) the puffed platelets from the unexpanded graphite particles.

Zhu et al use microwaves to assist in the expansion and reduction of dry graphite oxide powders [ Yanwu Zhu et al, "Microwave assisted expansion and reduction of graphite oxide for supercapacitors ]", Carbon, Vol.48, No. 7, P.2010, No. 6, p.2118-4. 2122 ]. Natural graphite is oxidized and intercalated to produce GO/GIC. This process suffers from the same disadvantages as the process of Drzal. The product is essentially microwave-expanded graphite oxide worms as incompletely separated/isolated graphene sheets.

The same problem is also associated with the study by Khavrel et al [ P.A. Khavrel et al, "Fluorinated microwaved extended graphite oxides: structured feedstocks and double layeractivities [ Fluorinated microwave expanded graphite oxide: structural features and double layer capacitance ] ", Fullerenes, Nanotubes and Carbon Nanostructures [ Fullerenes, Nanotubes and Carbon Nanostructures ], volume 24, 2016-stage 4 ]. Natural graphite is oxidized to produce Graphite Oxide (GO), which is then expanded to produce graphite oxide worms and platelets. The expanded GO platelets are then fluorinated to become fluorinated graphite oxide.

Chen et al use a combination of microwaves and chemical reducing agents to reduce graphene oxide in solution [ Wufeng Chen, Life Yan, Prakriti R. Bangal, "Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves ]", Carbon, Vol.48, No. 4, month 4 2010, p.1146-1152 ]. Graphene oxide sheets have been previously produced using the following means: the graphite is oxidized to produce graphite oxide, and individual graphene oxide sheets are then expanded and separated using a solution sonication of graphite oxide. Microwaves were used to help reduce the graphene oxide sheets already produced in the liquid solution, i.e., the mixed solution of N, N-dimethylacetamide and water (DMAc/H2O).

Similarly, Voiry et al [ Damien Voiry et al, "High-quality graphene via microwave reduction of solution-bulked graphene oxide ]", Science [ Science ], 2016, 9/23: 353, 6306, 1413 and 1416 ] graphite oxide powder was prepared from natural graphite. The graphite oxide is then dispersed in a liquid to form a solution (e.g., GO + water) that is expanded using ultrasound to produce graphene oxide sheets. The graphene oxide sheets are then dried and thermally reduced using a microwave oven to become Reduced Graphene Oxide (RGO). It should be noted that microwaves are not used to puff the graphite oxide; but rather, microwaves are used to thermally reduce the dried, ultrasonically expanded graphene oxide sheets.

Our group utilized a combination of microwaves and some strong acids to produce Graphene sheets directly from unoxidized and uninsulated graphite [ a.zhamu and Bor z.jang, "One-Step Production of Graphene Materials ]", U.S. patent application No. 13/317,100 (10/11/2011); now U.S. Pat. No. 8,747,623(06/10/2014) ]. Several years later, Matsumoto et al used a combination of microwaves and a specific set of oligomeric ionic liquids to produce graphene directly from natural graphite dispersed in such ionic liquids [ Michio Matsumoto, Yusuke Saito, Chiyoung Park, Takanori Fukushima, and Takuzo Aida, "ultra-high-throughput extrusion of graphite inter-prime 'single-layer' graphene using Ultrahigh-throughput expansion of graphite into original 'monolayer' graphene ], Nature Chemistry [ Nature Chemistry ], 7(2015) 730-. This group of ionic liquids is difficult and very expensive to produce, which is not conducive to large scale production.

Therefore, it is urgently required to have a graphene production method which requires a shortened process time and less energy consumption. The method must be capable of producing ultra-thin graphene sheets (primarily single-layer graphene or a mixture of single-layer and few-layer graphene sheets), and must not require additional steps to separate the graphene sheets from the unexpanded graphite powder.

Disclosure of Invention

The invention provides a method for preparing graphene from unexpanded graphite or graphite carbon capable of being expanded by microwaves. The method comprises the following steps: (a) feeding and feeding a powder of the microwave expandable unexpanded graphite or graphitic carbon onto a surface of a non-metallic solid substrate, wherein the powder is substantially in the shape of a ribbon having a first ribbon width and a first ribbon thickness; (b) moving the ribbon-shaped powder into a microwave applicator chamber, the microwave applicator chamber comprising a microwave power zone having a microwave application width and a microwave penetration depth, wherein the microwave application width is not less than the first ribbon width and the microwave penetration depth is not less than the first ribbon thickness, such that the entire ribbon-shaped powder receives and absorbs microwave power at a sufficient power level for a sufficient length of time to puff and separate the microwave-expandable unexpanded graphite or graphitic carbon into graphene sheets that occupy an expansion volume having a second width greater than the first ribbon width and a second thickness greater than the first ribbon thickness; and (c) removing the graphene sheets from the microwave chamber, cooling the graphene sheets, and collecting the graphene sheets (e.g., in a collection container).

In certain embodiments of the invention, the microwaveable, unexpanded graphite or graphitic carbon is selected from the group consisting of intercalated natural graphite, natural graphite oxide, natural graphite fluoride, intercalated synthetic graphite, synthetic graphite oxide, synthetic graphite fluoride, intercalated amorphous graphite, amorphous graphite oxide, amorphous graphite fluoride, intercalated Highly Oriented Pyrolytic Graphite (HOPG), HOPG oxide, HOPG fluoride, intercalated mesocarbon microbeads, oxidized mesocarbon microbeads, fluorinated mesocarbon, intercalated needle coke, needle coke oxide, fluorinated needle coke, intercalated carbon or graphite fibers, carbon oxide or graphite fibers, fluorinated carbon or graphite fibers, intercalated carbon nanofibers, carbon oxide nanofibers, carbon fluoride nanofibers, graphite nitride, chlorinated graphite, brominated graphite, iodinated graphite, or combinations thereof. The length of the starting graphite or graphitic carbon material is preferably less than 50 μm, more preferably less than 20 μm, even more preferably less than 10 μm, and most preferably less than 3 μm.

In certain embodiments, the powder of microwaveable unexpanded graphite or graphitic carbon further comprises 0.1 wt.% to 20 wt.% (preferably from 1 wt.% to 10 wt.%) of a dielectric heating promoter selected from water, polar organic molecules, inorganic dielectric materials, or combinations thereof. The powder must remain substantially solid. The graphite powder is solid even when the graphite material contains up to 20% by weight of water.

In certain preferred embodiments, the residence time of the microwaveable unexpanded graphite or graphitic carbon in the microwave power zone is from 10 seconds to 5 minutes, preferably from 30 seconds to 3 minutes.

Preferably, the first belt width is selected from the range from 1mm to 10cm and the first belt thickness is from 10nm to 3.8 cm. More preferably, the first belt width is selected from the range of 5mm to 5cm and the first belt thickness is from 1 μm to 2.5 cm. Most preferably, the first belt width is selected from the range of 1cm to 3cm and the first belt thickness is from 0.1mm to 1.0 cm.

We have found that if the powder fed into the microwave application chamber is confined to a width covered (attainable) by high power microwaves and the microwaves penetrate completely through the thickness of the powder sample, the powder may puff to the maximum extent so that most of the graphene planes puff completely and separate from each other. In contrast to the expanded graphite worms typically observed in prior art expansion processes of intercalated or oxidized graphite, the resulting product is a large number of substantially completely separated graphene sheets. Original powder (having a first width w₁And a first thickness t₁) Is greatly expanded to a volume (having a second width and a second thickness) that may be 300-1000 times the original powder volume. The ratio of the second width to the first width may be up to 500, more typically from 3 to 300,even more typically from 10 to 100. The thickness of the powder also increases significantly. Thus, the graphene sheets produced typically comprise at least 80% single layer graphene sheets, and more typically 90% single layer graphene. The remaining graphene sheets are mostly few-layer graphene with 2-10 hexagonal carbon atom planes.

It should be noted that the strip shape of the powder is only one example illustrating the best practice. It is not necessary to make the powder into a ribbon shape; it can be of any shape and size so long as the microwave power is substantially "focused" or confined within a microwave effective volume that completely surrounds the volume of powder fed into the microwave application zone. The powder fed into the microwave application zone may be of any shape or size as long as substantially all of the powder material is within the microwave effective volume, which may conveniently be designed as a generally rectangular volume having a width and thickness. The conveyor moves the powder into and out of the microwave application chamber in a manner such that the maximum powder sample width is less than the width of the microwave effective volume and the maximum powder sample thickness is less than the microwave penetration depth of the apparatus. Given the same microwave frequency (e.g., 2.45GHz or 915MHz), this penetration depth will vary from one expandable graphite material to another, but is typically from 1cm to 4 cm.

Accordingly, the present invention also provides a method of preparing graphene from microwaveable unexpanded graphite or graphitic carbon. The method comprises the following steps: (a) feeding and feeding a volume of the microwave expandable unexpanded graphite or graphitic carbon powder onto a surface of a non-metallic solid substrate, wherein the powder volume has a maximum width and a maximum thickness; (b) moving the powder into a microwave applicator chamber, the microwave applicator chamber comprising a microwave power zone having a microwave application width and a microwave penetration depth, wherein the microwave application width is not less than the maximum width of the powder volume and the microwave penetration depth is not less than the maximum thickness of the powder volume, such that the entire powder volume receives and absorbs microwave power at a sufficient power level for a sufficient length of time to puff and separate the microwave-expandable unexpanded graphite or graphitic carbon into graphene sheets that occupy an expanded volume having a second width greater than the maximum width and a second thickness greater than the maximum thickness; and (c) removing the graphene sheets from the microwave chamber, cooling the graphene sheets, and collecting the graphene sheets (e.g., in a collection container).

Depending on the type of starting graphite or graphitic carbon material, the graphene sheets made with the method of the present invention may include pristine graphene, graphene oxide with an oxygen content of less than 5 wt%, fluorinated graphene with less than 5 wt% fluorine, graphene with a carbon content of no less than 95 wt%, or functionalized graphene.

A microwave expandable powder of unexpanded graphite or graphitic carbon can be fed and moved into the microwave application chamber in a continuous or intermittent manner.

Optionally, the graphene sheets may be subjected to a mechanical shearing process to reduce the size of the graphene sheets. The mechanical shear treatment may include the use of air milling, air jet milling, ball milling, rotating blade mechanical shear, ultrasonic treatment, cavitation, or combinations thereof.

The present invention also provides a focused microwave-based system for producing graphene sheets from microwave expandable unexpanded graphite or graphitic carbon. The system comprises: (A) a solid powder feeding and guiding device capable of supplying and feeding a generally ribbon-shaped powder of microwaveable unexpanded graphite or graphitic carbon onto a solid substrate surface, wherein the ribbon-shaped powder has a first ribbon width and a first ribbon thickness; (B) a conveying apparatus in operative relationship with the powder feeding and guiding apparatus and supporting or containing the non-metallic solid substrate, moving the strip-shaped powder into at least one microwave applicator chamber (which may include a plurality of chambers); (C) a microwave power supply and power focusing subsystem that induces a microwave power zone in the microwave application chamber, wherein the microwave power zone has a microwave application width that is not less than the first ribbon width and a microwave penetration depth that is not less than the first ribbon thickness, such that the entire ribbon-shaped powder receives and absorbs microwave power to form graphene sheets via microwave thermally activated puffing and separation of the powder; and (D) a collector for collecting the graphene sheets.

The solid powder feeding and guiding means may comprise a feeder device selected from the group consisting of a vibratory feeder, a gravimetric feeder, a volumetric auger-type feeder, a syringe, a compressed air assisted feeder, a vacuum assisted feeder, a gravity feeder, a drum feeder, a wheel feeder, a skid, a chute, a conveyor feeder, or a combination thereof. Preferably, the solid powder feeding and guiding means comprises a guiding device for controlling the first belt width and a wiper for controlling the first thickness of the powder.

In a focused microwave based system, the microwave power supply and power focusing subsystem may include a plurality of microwave application chambers. The microwave power supply and power focusing subsystem may provide microwave power from 200W to 200kW, preferably from 400W to 100kW and more preferably from 700W to 50 kW.

In certain embodiments, the focused microwave-based system may further comprise means for introducing a protective gas atmosphere into the microwave applicator chamber, wherein the protective gas atmosphere comprises an inert gas, nitrogen, hydrogen, or a combination thereof.

The focused microwave-based system may further include a cooling zone in which the graphene sheets are cooled after microwave thermally activated bulking and separation of the powder.

In certain embodiments, the focused microwave-based system may further comprise a device to allow the exhaust gas to exit or a scrubber device to capture the exhaust gas.

Drawings

Fig. 1 shows a flow diagram of the most common prior art process for producing Graphite Intercalation Compounds (GICs) or graphite oxides.

Fig. 2(a) is a schematic diagram of a method of producing graphene sheets from a microwave expandable graphite material.

Fig. 2(B) is a schematic representation of a ribbon-shaped powder of expandable graphite material having an initial width (first width) that is expanded and puffed into graphene flake powder having an expanded powder width that is significantly greater than the initial powder width.

Fig. 3 is a schematic diagram of a focused microwave power-based apparatus for producing graphene materials.

Figure 4 TEM images of representative graphene sheets produced by the method of the invention.

Detailed Description

Carbon materials may be in a substantially amorphous structure (glassy carbon), highly organized crystals (graphite), or a full range of intermediate structures characterized by graphite crystallites and defects dispersed in amorphous matrices of various proportions and sizes. Typically, a graphite crystallite is composed of many graphene sheets or hexagonal planes of carbon atoms, which are bonded together in the c-axis direction (the direction perpendicular to the basal plane) by van der waals forces. These graphitic crystallites are typically on the micrometer or nanometer scale. The graphite crystallites are dispersed in or connected by crystalline defects or amorphous phases in graphite or graphitic carbon particles, which may be graphite flakes, carbon/graphite fiber segments, carbon/graphite whiskers, carbon/graphite nanofibers, disordered carbons (including soft and hard carbons), and the like.

A preferred embodiment of the present invention is a method of preparing a graphene material (also known as nano-graphene platelets, NGP) consisting essentially of one sheet of graphene plane or multiple sheets of graphene planes stacked and bonded together (typically, on average, up to 5 sheets per multi-layer platelet). Each graphene plane (also referred to as graphene sheet) contains a two-dimensional hexagonal structure of carbon atoms. Each platelet has a length and width parallel to the graphene plane and a thickness orthogonal to the graphite plane. By definition, NGPs have a thickness of 100 nanometers (nm) or less, with monolithic NGPs as thin as 0.34 nm. However, the method of the present invention produces graphene sheets typically comprising from 1 to 10 layers, or from 0.34nm to 3.4 nm. In many cases, the graphene sheets produced are predominantly single-layer graphene. The length and width of the NGPs are typically between 200nm and 20 μm.

As shown in fig. 1, prior art chemical methods for graphene preparation typically involve immersing natural graphite powder in a mixture of concentrated sulfuric acid, nitric acid, and an oxidizing agent such as potassium permanganate or sodium perchlorate. The chemical intercalation/oxidation reaction typically takes 5-120 hours to complete. Once the reaction is complete, the slurry is subjected to repeated steps of rinsing and washing with water, and then to a drying treatment to remove the water. The dried powder is commonly referred to as Graphite Intercalation Compound (GIC) or Graphite Oxide (GO). The GO/GIC is then subjected to a thermal shock treatment, most typically accomplished by exposing the GIC/GO to a furnace preset to a temperature of typically 800-. Such thermal shock furnace-based operations are less energy efficient, typically result in the formation of expanded graphite worms (requiring further mechanical shearing or sonication to produce isolated graphene sheets), and are less prone to the formation of ultra-thin graphene sheets (single or few layers).

In certain embodiments, referring to fig. 2(a) and 2(B), as illustrative examples, the method of the present invention comprises:

(a) feeding a microwave expandable unexpanded graphite or graphitic carbon powder (18) onto a non-metallic solid substrate surface (e.g., supported by or contained in a conveyor belt (12)) using a powder feeder (16), wherein the powder is substantially in the shape of a belt having a first belt width, a first belt thickness, and a length (the length of the belt being "dynamic" in that the apparatus continuously feeds and moves the powder into a microwave applicator chamber while the expanded powder is continuously removed from the microwave heating zone);

(b) moving the ribbon-shaped powder into a microwave applicator chamber (a first portion of the powder treatment zone 20) that includes a microwave power zone (e.g., under a focused microwave guidance and power limiting subsystem 24) having a microwave application width and a microwave penetration depth, wherein the microwave application width is no less than the first ribbon width and the microwave penetration depth is no less than the first ribbon thickness, such that the entire ribbon-shaped powder receives and absorbs microwave power at a sufficient power level for a sufficient length of time to puff and separate the microwave-expandable unexpanded graphite or graphitic carbon (powder 18) into graphene sheets (28) that occupy an expanded volume having a second width greater than the first ribbon width and a second thickness greater than the first ribbon thickness; and

(c) the graphene sheets are removed from the microwave chamber, the graphene sheets are cooled (e.g., in cooling zone 26), and the cooled graphene sheets are collected using a collector (e.g., vacuum-assisted collection vessel 32) (30). The moving conveyor (12) is driven by a pair of rollers (14a and 14 b). At least one of the two rollers is powered by a motor.

The non-expanded graphite or graphitic carbon that is microwave expandable may be selected from intercalated natural graphite, oxidized natural graphite, fluorinated natural graphite, intercalated synthetic graphite, oxidized synthetic graphite, fluorinated synthetic graphite, intercalated amorphous graphite, oxidized amorphous graphite, fluorinated amorphous graphite, intercalated Highly Oriented Pyrolytic Graphite (HOPG), oxidized HOPG, fluorinated HOPG, intercalated meso-carbon microspheres, oxidized meso-carbon microspheres, fluorinated meso-carbon, intercalated needle coke, oxidized needle coke, fluorinated needle coke, intercalated carbon or graphite fibers, carbon oxide or graphite fibers, intercalated carbon nanofibers, carbon oxide nanofibers, carbon fluoride nanofibers, graphite nitride, other halogenated graphites, or combinations thereof. The length of the starting graphite or graphitic carbon material is preferably less than 50 μm, more preferably less than 20 μm, even more preferably less than 10 μm, and most preferably less than 3 μm.

There is no such versatile and effective prior art method for preparing ultra-thin graphene sheets (primarily single-layer graphene or a mixture of single-layer graphene and few-layer graphene) from such a variety of different types of graphite materials.

In certain embodiments, the microwaveable unexpanded graphite or graphitic carbon powder further comprises from 0.1 wt.% to 20 wt.% (preferably from 1 wt.% to 10 wt.%) of a material selected from water, polar organic molecules (e.g., acetone, alcohols, parylene, etc.), inorganic dielectric materials (e.g., various metal oxides, nitrides, borides, or carbides; salts, e.g., NaH₂PO₄Zeolite, etc.) or combinations thereof. The powder must remain substantially solid. Even if the graphite material contains 2 or more0% by weight of water, the graphite powder is still solid.

Another embodiment of the present invention is a focused microwave-based system or apparatus for producing graphene sheets from microwave expandable unexpanded graphite or graphitic carbon. As an illustrative example, referring to fig. 3, the system comprises:

(A) solid powder feeding and guiding devices (e.g., vacuum assisted feeder 52 and reservoir 50 for expandable material) capable of supplying and feeding a generally ribbon-shaped powder of microwaveable unexpanded graphite or graphitic carbon onto a non-metallic solid substrate surface (of conveyor belt 58), wherein the ribbon-shaped powder has a first ribbon width and a first ribbon thickness;

(B) a conveyor apparatus 58 in operative relationship with the powder feeding and guiding device and supporting or containing the non-metallic solid substrate, the conveyor apparatus moving the strip-shaped powder into at least one microwave applicator chamber (which may include multiple chambers);

(C) a microwave power supply and power focusing subsystem (e.g., including a microwave generator 56 and a transmitter 55) that induces a microwave power zone 54 in the microwave application chamber, wherein the microwave power zone has a microwave application width that is not less than the first ribbon width and a microwave penetration depth that is not less than the first ribbon thickness, such that the entire ribbon-shaped powder receives and absorbs microwave power to form graphene sheets via microwave thermally activated puffing and separation of the powder; and

(D) a collector 66 for collecting graphene sheets.

At the far left of FIG. 3 is vacuum assistA feeder 52 which draws expandable graphite or graphitic carbon powder from the reservoir 50 and feeds it continuously or intermittently onto the non-metallic surface of the conveyor belt 58. A protective gas inlet 60 is located near the inlet of the microwave power zone 54. The gas inlet will supply a protective gas (inert gas, N)₂、H₂Etc.) into a microwave power zone (or powder heating zone).

The microwave power zones are enabled or generated by a microwave generator 56 and transmitter 55, which may limit or confine the maximum power within a generally rectangular zone. The depth of this rectangular microwave power zone should be equal to or slightly greater than the maximum microwave penetration depth, which is typically 1cm to 4cm for expandable graphite or graphitic carbon materials. The powder of expandable graphite or graphitic carbon enters and exits the microwave power zone at a velocity that allows the powder material to be exposed to microwave power for a sufficient exposure time (or sufficient residence time from the entry point to the exit point) to undergo effective puffing and separation of the graphene planes. The graphene sheets produced are then moved into a cooling zone 62 adjacent to a microwave power zone (or heating zone) and then collected by a collector 66. There may be an exhaust gas outlet 64 and a scrubber system for capturing gaseous species.

It should be noted that the powder fed onto the non-metallic substrate surface of the conveyor need not be in the shape of a strip or any particular shape. However, the maximum width of the shape should not exceed the maximum width of the effective microwave power zone to ensure that the entire powder volume is exposed to the required microwave power. Likewise, the powder shapes may have varying thicknesses, but the thickness should not exceed the maximum penetration depth for a given microwave frequency in a given graphite material.

The frequencies of microwave or radio frequency radiation that may be used to practice the present invention are not necessarily limited to 2.45GHz or 0.915GHz for use in a domestic microwave oven. Preferably, the frequency is between 0.9 and 20GHz, and more preferably between 2 and 10 GHz.

The starting graphite material may be selected from the group consisting of intercalated, oxidized, halogenated (including fluorinated, chlorinated, brominated or iodinated) or nitrated versions of natural graphite, amorphous graphite (a graphite material containing micron-sized graphite crystallites, typically 0.1-1.0 μm), synthetic graphite, highly oriented pyrolytic graphite, mesocarbon microbeads, graphitized mesocarbon, needle coke, carbon fibers, graphite fibers, carbon nanofibers, graphite fluorinated graphite, chemically modified graphite, expanded graphite, or combinations thereof.

Graphite Oxide (GO) may be obtained by immersing a starting graphite or graphite carbon powder or filament (e.g. synthetic graphite powder, soft carbon, hard carbon, carbon nanofibers, multiwalled carbon nanotubes, graphite fibers, needle coke, etc.) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid, nitric acid and potassium permanganate) in a reaction vessel at a desired temperature for a period of time (typically from 0.5 to 96 hours, depending on the nature of the starting material and the type of oxidant used). Preferably, the oxidation is carried out to such an extent that the oxygen content in the graphite or graphitic carbon material is preferably not less than 20 wt%. The oxygen content is more preferably greater than 30 wt%, most preferably greater than 40 wt%. Followed byThe washing is repeated, and then dried to obtain graphite oxide or graphite oxide carbon. Thereafter, if desired, by using other chemical groups (e.g. -Br, NH)₂Etc.) to replace-OH groups in the graphite oxide, and convert the resulting graphite oxide into various functionalized graphite materials.

Fluorinated graphene or fluorinated graphene is used herein as an example of a halogenated graphene material group. The fluorinated graphene may be obtained by microwave-induced puffing of graphite fluoride. F₂Interaction with graphite at high temperature to give covalent graphite fluoride (CF)_nOr (C)₂F)_nWhile forming Graphite Intercalation Compound (GIC) C at low temperature_xF (x is more than or equal to 2 and less than or equal to 24). In (CF)_nThe carbon atoms are sp3 hybridized and thus the fluorocarbon layer is corrugated, consisting of trans-linked cyclohexane chairs. In (C)₂F)_nOnly half of the C atoms are fluorinated and each pair of adjacent carbon sheets are linked together by a covalent C — C bond. Systematic studies of the fluorination reaction show that the resulting F/C ratio depends to a large extent on the fluorination temperature, the partial pressure of fluorine in the fluorination gas, and the physical properties of the graphite precursor, including graphitization degree, particle size and specific surface area. In addition to fluorine (F)₂) In addition, other fluorinating agents may be used, but most of the prior art references relate to the use of F₂The gas is fluorinated (sometimes in the presence of fluoride).

The graphene nitride can be formed at a lower temperature by a hydrothermal method; for example by sealing graphite oxide and ammonia in an autoclave and then heating to 150 c to 250 c. The nitrogen content or the combined amount of oxygen and nitrogen in the material is preferably at least 20%, more preferably at least 30%, most preferably more than 40%.

Acids (e.g., sulfuric acid) are not the only type of intercalating agent (intercalant) that can penetrate into the spaces between graphene planes to obtain GIC. Many other types of intercalation agents such as alkali metals (Li, K, Na, Cs and alloys or eutectics thereof), metal salts (e.g., metal trichloride), and many other organic or inorganic substances (e.g., tetrabutylammonium) can be used to intercalate graphite into 1-stage, 2-stage, 3-stage, and the like. The n-order represents one intercalant layer per n graphene planes. For example, 1 stage potassium intercalated GIC means one K layer per graphene plane; alternatively, there may be a layer of graphene (referred to herein as "G") between two adjacent planes of intercalating agent species (referred to as "A") in the G/A/G/A/G/A/G sequence, where G is the graphene plane and A is the intercalating agent plane. A second order GIC will have the sequence GG/A/GG/A/GG/A/GG …, a third order GIC will have the sequence GGG/A/GGG/A/GGG …, and so on. These GICs can then be fed to a microwave power zone for puffing. To prepare ultra-thin graphene sheets, the GIC preferably comprises only 1-or 2-stage compounds, most preferably only 1-stage compounds.

It is clear that the required microwave exposure time may be less than 5 minutes, typically less than 3 minutes, or even less than 1 minute. The microwave exposure step may be followed by a step of subjecting the resulting expanded mass of graphene sheets to a mechanical shearing process to produce smaller (shorter or narrower) graphene sheets. Mechanical shear processing includes the use of air milling, air jet milling, ball milling, rotating blade mechanical shear, ultrasonic processing, cavitation, or combinations thereof.

The method and related microwave equipment of the invention can prepare single-layer graphene sheets. In many examples, the graphene material produced comprises at least 80% or 90% single-layer graphene sheets (with the remainder of the graphene sheets being few or no greater than 10 layers). In some samples, the graphene sheets are predominantly monolayer. The fabricated graphene may include pristine graphene, graphene oxide having an oxygen content of less than 5 wt%, graphene fluoride having a fluorine content of less than 5 wt%, graphene having a carbon content of not less than 95 wt%, or functionalized graphene.

The following examples are intended to provide the best modes of practicing the invention and should not be construed as limiting the scope of the invention:

example 1: preparation of graphite oxide using a modified Hummers process

Graphite oxide is prepared by oxidizing natural graphite flakes with sulfuric acid, sodium nitrate and potassium permanganate according to Hummers' method [ U.S. Pat. No. 2,798,878, 7/9/1957 ]. In this example, for every 1g of graphite, a mixture of 22ml of concentrated sulfuric acid, 2.8 g of potassium permanganate and 0.5 g of sodium nitrate is used. Graphite flakes were immersed in the mixture solution and the reaction time was about 5 hours at 35 ℃. It is important to note that potassium permanganate should be gradually added to the sulfuric acid in a well controlled manner to avoid overheating and other safety issues. After the reaction was complete, the sample was then washed repeatedly with deionized water until the filtrate had a pH of about 5. The dried product is sulfuric acid intercalated Graphite (GIC) or graphite oxide.

The GIC particles were then exposed to microwave power (25kW) at 2.45GHz for 45 seconds to obtain isolated/isolated graphene sheets, which were predominantly single-layer graphene. The ribbon-shaped powder was 0.85cm wide and 0.75cm thick before moving into the microwave power zone. After 45 seconds of microwave exposure, the tape expanded to 62cm width and 4.6cm thickness. Fig. 4 shows TEM images of representative graphene sheets produced.

Example 2: production of isolated graphene sheets from needle coke

Intercalated graphitic carbon was prepared by an electrochemical intercalation procedure using commercially available needle coke (Jinzhou Petrochemical Co). A sample of surface treated needle coke (needle coke filament) was prepared by immersing the filament in concentrated sulfuric acid for 0.5 hours to remove the hard carbon skin. Needle-shaped coke filaments with an average length of 35 μ M were used as anode material and 1,000mL of liquid solution electrolyte (typically 1M alkali metal salt in organic solvent) was used. A mixture of Ethylene Carbonate (EC) and Propylene Carbonate (PC) was used as solvent. The alkali metal salts used in this example respectively include lithium perchlorate (LiClO)₄) And sodium perchlorate (NaClO)₄)。

The anode support member was a stainless steel plate and the cathode was graphite foam having a diameter of about 4cm and a thickness of 0.2cm, which was impregnated with lithium or sodium. A separator (glass fabric) is used to separate the cathode plate from the needle coke filaments and to press these particles down against the anode support member to ensure that the needle coke filaments are in electrical connection with the anode support member to act as an anode. The electrodes, electrolyte and separator were housed in a Buchner-type funnel to form an electrochemical cell. The anode support element, cathode and separator are porous to allow the intercalation (contained in the electrolyte) to saturate the needle coke and pass through the cell from top to bottom.

At a current of 0.5 Amp (Current Density of about 0.04 Amp/cm)²) The needle coke filaments were subjected to an electrochemical loading treatment (i.e., loading of alkali metal ions into the graphene interplanar spaces in the needle coke filaments) for 2-5 hours at a battery voltage of about 4-6 volts. These values may vary with battery configuration and composition. After the electrochemical loading treatment, the resulting intercalated filaments were washed with water and dried.

Subsequently, some of the intercalated compound is subjected to microwave heating. Various samples were collected, their morphology was studied by SEM, TEM and Raman spectroscopy (Raman) observations, and their specific surface area was measured by the well-known BET method. It was found that the BET specific surface area was from 675 to 1,050m²This indicates that the graphene sheets are predominantly monolayer (> 80%) and few layer varieties. The first powder tape width was 4.8cm and the first tape thickness was about 2 cm. After puffing, the graphene powder was 100cm wide (confined between two teflon plate based walls) and 9.5cm thick.

Example 3: preparation of discrete GO sheets from graphite fibers

Chopped graphite fibers having an average diameter of 12 μm were used as starting materials, which were immersed in a mixture of concentrated sulfuric acid, nitric acid and potassium permanganate as chemical intercalators and oxidants to produce fiber-based Graphite Intercalation Compounds (GICs). The starting material was first dried in a vacuum oven at 80 ℃ for 24 h. Then, a mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (at a weight ratio of 4:1: 0.05) was slowly added to the three-neck flask containing the fiber section with appropriate cooling and stirring. After 12 hours of reaction, the acid treated graphite fibers were filtered and washed thoroughly with deionized water until the pH level of the solution reached 6. After drying overnight at 60 ℃, the resulting graphite oxide fiber powder in the form of a tape having a first width of 5cm and a first thickness of 2.5cm was subjected to microwave treatment (25kW for 30 seconds). The resulting expanded powder comprises all single or few graphene sheets, and no unexpanded graphite fiber powder.

For comparison, two samples of the same graphite oxide fiber powder were subjected to the same microwave treatment conditions; one powder sample (sample 3b) had a first width of 11cm and a first thickness of 2.5cm, and the other sample (sample 3c) had a first width of 5cm and a first thickness of 4.5 cm. We observed that sample 3b had approximately 20% unexpanded graphite fibers after exposure to microwaves, possibly due to some powder being located outside the focused microwave power region. Sample 3c had approximately 15% unexpanded graphite fibers, which is probably due to the inability of the microwaves to penetrate the entire depth of the powder.

Example 4: preparation of single-layer graphene sheets from mesocarbon microbeads (MCMB)

Mesocarbon microbeads (MCMB) are supplied by China Steel Chemical co. The material has a density of about 2.24g/cm³And a median particle size of about 16 μm. In one example, MCMB (10 grams) was intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate in a 4:1:0.05 ratio) for 48-96 hours. After the reaction was complete, the mixture was poured into deionized water and filtered. The intercalated MCMB was repeatedly washed in a 5% solution of HCl to remove most of the sulfate ions. The sample was then repeatedly washed with deionized water until the pH of the filtrate was not less than 4.5. The slurry is then subjected to microwave treatment. TEM, atomic force microscopy and BET studies show that most GO sheets are single layer graphene when oxidized for more than 72 hours, and 2 or 3 layers graphene when oxidized for 48 to 72 hours. For oxidation treatment times of 48-96 hours, the GO sheets contain oxygen proportions of about 35 wt% to 47 wt%.

Example 5: preparation of fluorinated graphene nanosheet

We have used several methods to prepare GF, but only one method is described herein as an example. In a typical procedure, highly oxidized artificial graphite is subjected to fluorination with chlorine trifluoride vapor at 100 ℃ for 10 hours, resulting in the formation of lightly fluorinated graphite. The pre-cooled Teflon reactor is filled with 20-30mL of liquid pre-cooled ClF₃The reactor was closed and cooled to liquid nitrogen temperature. Then, 1g of lightly fluorinated graphite was placedInto a container having a ClF supply₃The gas enters the reactor and is located in an aperture within the reactor. Form a product with approximate formula C in 7-10 days₂F as a grey beige product.

Then, these C are combined₂The F particles were exposed to microwave power (10W) at 2.45GHz for 60 seconds to obtain fluorinated graphene sheets (>90% monolayer fluorinated graphene sheets). The ribbon-shaped powder was 1.5cm wide and 1.5cm thick before moving into the microwave power zone. After 1 minute microwave exposure, the tape expanded to 44cm width and 4.6cm thickness.

Example 6: preparation of nitrogenated graphene nanosheet and porous graphene structure

The graphite oxide (as prepared in example 1) was converted to graphite nitride using a hydrothermal method. The graphite oxide particles were sealed in an autoclave with ammonia and then the temperature was raised to 220 ℃. The reaction was allowed to proceed for 6 hours to obtain graphite nitride particles. These particles were then exposed to microwave power (700W) at 2.45GHz for 60 seconds to obtain graphene nitride sheets. The ribbon-shaped powder was 1.1cm wide and 1cm thick before moving into the microwave power zone. After 1 minute microwave exposure (1 minute dwell time), the tape expanded to 39cm width and 3.5cm thickness.

Example 7: preparation of various graphite oxides and graphitic carbon materials

Several graphite oxide carbon materials were prepared according to the same procedure as used in example 1, but the starting graphite materials were powders of Highly Oriented Pyrolytic Graphite (HOPG), natural graphite powder, pitch-based graphite fiber, vapor grown carbon nanofibers (VG-CNF), multiwall carbon nanotubes (MW-CNTs), and amorphous graphite, respectively. In each sample, up to 20% water or alcohol was added as a dielectric heating promoter to the graphite oxide material. We have surprisingly observed that an amount (e.g. 5%) of the dielectric heating promoter can significantly increase the amount of single-layer graphene sheets. The specific surface area is typically from 260-450m²Increase in/g to 650-950m²/g。

Example 8: preparation of graphitic carbon from soft carbon particles

Soft carbon particles are prepared from a liquid crystalline aromatic resin. The resin was ground in a mortar and in N₂Calcining at 900 deg.C for 2h in atmosphere to prepareGraphitized carbon, also known as soft carbon. The soft carbon particles were subjected to intercalation and microwave exposure treatment in a similar manner to example 1. The resulting product is predominantly small monolayer graphene sheets plus some amorphous carbon powder.

Example 9: preparation of graphitic carbon from petroleum pitch-derived hard carbon particles

Asphalt samples (A-500 from Ashland Chemical Co.) were carbonized at 900 ℃ for 2 hours and then at 1,200 ℃ for 4 hours. In order to remove the carbon skin of the pitch-based hard carbon particles, the hard carbon particles were surface-treated with an aqueous KOH solution (5% concentration). The hard carbon particles were subjected to intercalation and microwave exposure treatment in a similar manner to example 1. The resulting product is predominantly small monolayer graphene sheets plus some amorphous carbon powder.

Claims

1. A process for producing graphene from microwaveable unexpanded graphite or graphitic carbon, the process comprising:

(a) feeding and feeding a powder of the microwave expandable unexpanded graphite or graphitic carbon onto a non-metallic solid substrate surface, wherein the powder is in the general shape of a strip having a first strip width and a first strip thickness;

(b) moving the ribbon-shaped powder into a microwave applicator chamber, the microwave applicator chamber comprising a microwave power zone having a microwave application width and a microwave penetration depth, wherein the microwave application width is not less than the first ribbon width and the microwave penetration depth is not less than the first ribbon thickness, such that the entire ribbon-shaped powder receives and absorbs microwave power at a sufficient power level for a sufficient length of time to puff and separate the microwave-expandable unexpanded graphite or graphitic carbon to produce graphene sheets that occupy an expanded volume having a second width greater than the first ribbon width and a second thickness greater than the first ribbon thickness; and

(c) moving the graphene sheet out of the microwave chamber, cooling the graphene sheet, and collecting the graphene sheet.

2. The process of claim 1, wherein the microwave expandable unexpanded graphite or graphitic carbon is selected from the group consisting of intercalated natural graphite, natural graphite oxide, natural graphite fluoride, intercalated synthetic graphite, synthetic graphite oxide, synthetic graphite fluoride, intercalated amorphous graphite, amorphous graphite oxide, amorphous graphite fluoride, highly oriented pyrolytic graphite intercalation (HOPG), HOPG oxide, HOPG fluoride, intercalated mesocarbon microbeads, oxidized mesocarbon microbeads, fluorinated mesocarbon, intercalated needle coke, needle coke oxide, needle coke fluoride, intercalated carbon or graphite fibers, carbon oxide or graphite fibers, carbon fluoride or graphite fibers, intercalated carbon nanofibers, carbon oxide nanofibers, carbon fluoride nanofibers, graphite nitride, chlorinated graphite, brominated graphite, graphite iodide, or combinations thereof.

3. The process of claim 1 wherein the microwave expandable unexpanded graphite or graphitic carbon powder further comprises 0.1 wt.% to 20 wt.% of a dielectric heating promoter selected from water, polar organic molecules, inorganic dielectric materials, or combinations thereof.

4. The process of claim 1 wherein the residence time of the microwave expandable unexpanded graphite or graphitic carbon in the microwave power zone is from 10 seconds to 5 minutes.

5. The method of claim 1, wherein the residence time is from 30 seconds to 3 minutes.

6. The method of claim 1, wherein the first belt width is selected from the range of 1mm to 10cm and the first belt thickness is from 10nm to 3.8 cm.

7. The method of claim 1, wherein the first belt width is selected from the range of 5mm to 5cm and the first belt thickness is from 1 μ ι η to 2.5 cm.

8. The method of claim 1, wherein the first belt width is selected from the range of 1cm to 3cm and the first belt thickness is from 0.1mm to 1.0 cm.

9. The process of claim 1 wherein the microwave expandable unexpanded graphite or graphitic carbon powder is fed and moved into the microwave application chamber in a continuous or batch manner.

10. The method of claim 1 wherein the ratio of the second width to the first ribbon width is from 3 to 300.

11. The method of claim 1, wherein the graphene sheets are subjected to a mechanical shearing process to produce smaller graphene sheets.

12. The method of claim 11, wherein the mechanical shear treatment comprises using air milling, air jet milling, ball milling, rotating blade mechanical shear, sonication, cavitation, or combinations thereof.

13. The method of claim 1, wherein the graphene comprises a single layer graphene sheet.

14. The method of claim 1, wherein the graphene comprises at least 80% single-layer graphene sheets.

15. The method of claim 1, wherein the graphene comprises pristine graphene, graphene oxide having an oxygen content of less than 5 wt%, graphene fluoride having less than 5 wt% fluorine, graphene having a carbon content of no less than 95 wt%, or functionalized graphene.

16. A process for producing graphene from microwaveable unexpanded graphite or graphitic carbon, the process comprising: (a) feeding and feeding a volume of a microwaveable, unexpanded graphite or graphitic carbon powder onto a surface of a solid substrate, wherein the powder volume has a maximum width and a maximum thickness; (b) moving the powder into a microwave applicator chamber, the microwave applicator chamber comprising a microwave power zone having a microwave application width and a microwave penetration depth, wherein the microwave application width is not less than the maximum width of the powder volume and the microwave penetration depth is not less than the maximum thickness of the powder volume, such that the entire powder volume receives and absorbs microwave power at a sufficient power level for a sufficient length of time to puff and separate the microwave-expandable unexpanded graphite or graphitic carbon into graphene sheets that occupy an expanded volume having a second width greater than the maximum width and a second thickness greater than the maximum thickness; and (c) removing the graphene sheets from the microwave chamber, cooling the graphene sheets, and collecting the graphene sheets.

17. A focused microwave-based system for producing graphene sheets from microwaveable, unexpanded graphite or graphitic carbon, the system comprising:

(a) a solid powder feeding and directing device that supplies and feeds the ribbon-shaped powder of microwave expandable unexpanded graphite or graphitic carbon onto a non-metallic solid substrate surface, wherein the ribbon-shaped powder has a first ribbon width and a first ribbon thickness;

(b) a conveying apparatus in operative relationship with the powder feeding and guiding apparatus and supporting or containing the non-metallic solid substrate, moving the strip-shaped powder into at least one microwave applicator chamber;

(c) a microwave power supply and power focusing subsystem that induces a microwave power zone in the microwave application chamber, wherein the microwave power zone has a microwave application width that is not less than the first ribbon width and a microwave penetration depth that is not less than the first ribbon thickness, such that the entire ribbon-shaped powder receives and absorbs microwave power to form graphene sheets via microwave thermally activated puffing and separation of the powder; and

(d) a collector for collecting the graphene sheets.

18. The focused microwave based system as claimed in claim 17, wherein the solid powder feeding and guiding device comprises a feeder apparatus selected from the group consisting of a vibratory feeder, a gravimetric feeder, a volumetric auger-type feeder, a syringe, a compressed air assisted feeder, a vacuum assisted feeder, a gravity feeder, a drum feeder, a wheel feeder, a skid, a chute, a conveyor feeder, or combinations thereof.

19. The focused microwave based system according to claim 17, wherein the solid powder feeding and guiding device comprises a guiding apparatus for controlling the first ribbon width and a wiper for controlling the first thickness of the powder.

20. The focused microwave based system as claimed in claim 17 wherein the microwave power supply and power focusing subsystem includes a plurality of microwave application chambers.

21. The focused microwave based system according to claim 17, where the microwave power supply and power focusing subsystem provides microwave power from 200W to 200 kW.

22. The focused microwave based system according to claim 17, further comprising means for introducing a protective gas atmosphere into the microwave applicator chamber, wherein the protective gas atmosphere comprises an inert gas, nitrogen, hydrogen, or combinations thereof.

23. The focused microwave based system of claim 17, further comprising a cooling zone, wherein the graphene sheets are cooled after microwave thermally activated puffing and separation of the powder.

24. The focused microwave based system according to claim 17, further comprising means for allowing exhaust gas to exit or scrubber means for capturing exhaust gas.